I've downloaded the public beta and written a few thousand words with it so far (unfortunately, most of them were lost when the program crashed, but hey, it's a beta). I've been a Mac user since 1988 and for most of that time have used Word for long technical documents and WriteNow for fiction and esays. While a Word-killer would be nice, what I really want is a replacement for WriteNow, which is getting perilously old and is one of the only reasons I still run Classic. Along those lines, here are the features Nisus Writer Express must have if I'm to switch to it as my production writing tool (and believe me, I want to make the switch):

Performance. The last revision of WriteNow was released 9 years ago, which means that it's still the same 68k assembler it always was, now being run inside Clasic. Yet it feels much snappier than NWX. I know that Cocoa adds overhead, but I feel like a 500MHz G4 should be able to keep up with my typing. It's a quirk of mine.

Footnotes and endnotes. I'm no David Foster Wallace, but even so, I still have need to add citations from time to time. Integrating with Endnote or any of its competitors would be fantastic.

Simple styles. At least copyable rulers and fonts a la TextEdit, although a full stylesheet implementation would be best, even if it's not completely configurable.

And, aside from that, being able to natively read WriteNow documents would make my day, although that's not a requirement.

I think you guys are off to a solid start and I'll be following the beta closely. Good luck!